Centrifugal pumps are known. Two types of pumps can de distinguished: pumps with single bent rotor blades, such as shown in EP2236836A2 and JP 8 284 885, and pumps with double bent rotor blades. Examples thereof are known from European patent application EP 1 903 216 A1 and EP1906029. This document relates to problems and solutions thereof associated with double bent rotor blades.
An example of such a centrifugal pump with double bent rotor blades is depicted in FIGS. 1 and 2.
FIGS. 1 and 2 schematically depict a centrifugal pump 1, comprising a pump housing 2 shaped like a volute (spiral casing). The pump housing 2 has a circumferential wall 3 and a spout-shaped outlet 5 attached tangentially to the circumferential wall 3 of the pump housing 2. The junction between the inner surface of the tangential outlet 5 and the inner surface of the circumferential wall 3 of the pump housing 2 defines what is known as a cutwater 4. The pump housing 2 also has an axial inlet 6.
A rotor 7 is attached in the pump housing 2 such that it may rotate about an axial rotation axis A. The rotor 7 has a central boss 9 which may be fastened to a drive shaft (not shown). A shaft shield 11 extends from the central boss 9. The shaft shield 11 forms a first wall for delimiting the flow within the rotor 7. Axially set apart from the shaft shield 11, the rotor has a suction shield 12 which defines a second wall for delimiting the flow within the rotor 7. The suction shield 12 has an axial supply 14 which is aligned with the axial inlet of the pump housing 2.
A plurality (three in FIGS. 1 and 2) of rotor blades 15 are fastened between the shields 11, 12. In this illustrative embodiment, the rotor 7 comprises three rotor blades 15. The rotor blades 15 each extend substantially radial to the rotation axis A. Each rotor blade 15 comprises a radial inner end 18 (leading edge) and a radial outer end 17 (trailing edge). The radial outer ends 17 and radial inner ends 18 run from the shaft shield 11 to the suction shield. Between the radial outer ends 17 of the rotor 7 and the inner surface of the circumferential wall 3 of the pump housing 2 there is a circumferential channel 19. The circumferential channel 19 has a passage surface area which increases somewhat in the circumferential direction from the cutwater 4 toward the outlet 5.
The characteristic dimensions of the centrifugal pump 1 are indicated in FIGS. 1 and 2. These characteristic dimensions largely determine the characteristics of the pump. The rotor 7 has an outermost diameter Dw which is defined by the radial outer edges of the shields 11, 12. The rotor 7 has a width Bw extending between the mutually facing surfaces of the shaft shield 11 and the suction shield 12. The axial supply 14 of the rotor 7 defines a suction diameter Dz. An inlet pipe can be connected to the axial inlet 6 of the pump housing 2. The centrifugal pump 1 also has what is known as a spherical passage Bol which is defined by the diameter of the largest sphere able to pass between the rotor blades (indicated in FIG. 1).
The rotor blades 15 are double bent rotor blades, which means that the rotor blade is curved in a first direction from the radial inner end 18 (leading edge) to the radial outer end 17 (trailing edge) and is curved in a second direction perpendicular to the first direction. Also indicated in FIG. 1 is a throat diameter Dk which is defined by the narrowest passage of the spout-shaped outlet 5 of the pump housing 2. Said narrowest passage is located in proximity to the cutwater 4. The spout-shaped outlet 5 also has a press diameter Dp located at the tip thereof. The distance, extending parallel to the centre line B of the outlet 5, between the crossing 4 and the level of the rotation axis A is indicated by the parameter T. The thickness of the circumferential channel 19 at the location of the cutwater 4 is represented in FIG. 1 by V.
During operation, the rotor rotates about the rotation axis A. Between the rotor blades 15, the mass to be pumped is forced radially outward into the pump housing 2 under the influence of centrifugal forces. Said mass is then entrained in the circumferential direction of the pump housing 2 toward the tangential outlet spout 5 of the pump housing 2. The pumped mass which, after leaving the rotor 7, is entrained in the circumferential direction of the pump housing 2 flows largely out of the tangential outlet of the pump housing 2. A small amount of the entrained mass recirculates, i.e. flows along the cutwater back into the pump housing 2.
FIG. 3 shows a perspective view of an example of the rotor 7 described above with reference to FIGS. 1 and 2.
Said centrifugal pump 1 can be used in dredging operations. If the centrifugal pump 1 is located on board a dredging ship, such as a cutter suction dredger or hopper suction dredger the centrifugal pump 1 has to fetch a loose mixture of substances, possibly including soil, stones and/or pebbles, from the sea floor. The main characteristics of a centrifugal pump used in dredging operations are 1) suction capacity, 2) durability and 3) spherical passage Bol (see FIG. 1). In use, the mixture of substances, possibly including stones and/or pebbles, flows through the centrifugal pump 1. In order to prevent blockage, said stones and/or pebbles have to be able to pass through the centrifugal pump 1 (Bol). A wide centrifugal pump 1 having few blades is suitable for this purpose. However, by widening the centrifugal pump 1 and reducing the number of rotor blades 15, the suction characteristics and durability of the centrifugal pump 1 are adversely affected.
DE 101 49 648 A1 describes an example of a pump with double bent rotor blades. This pump is of a different design than the pumps described above, i.e. the leading edge does not run from the shaft shield to the suction shield, but originates at a central shaft and does not end at the suction shield. Another centrifugal pump is shown in JP 8 284 885 A.
An object of the invention is to provide an improved centrifugal pump, which combines a relatively high suction capacity, with an improved durability and spherical passage.